Effect of titanium on the martensitic transformation temperature of Cu-14Al-4Ni Shape Memory Alloy rapidly solidified

Full text: The copper based shape memory alloys (SMAs) have been attracting attention of scientists and researchers due to low cost, wide range of transformation temperature, high thermal stability and low hysteresis level, as well as their easy production. Among them, the Cu-14Al-4Ni (wt.%) offers a greater potential for engineering applications, such as actuators, sensors or dampers for aeronautic purposes, because of its high transformation temperatures and better thermal stability comparing to the Cu-Zn-Al. However, due to its susceptibility to brittle intergranular fracture, multiple nucleation of cracks on the grain boundaries occur, which limits its industrial application. The present research aimed to combine rapid solidification process and addition of different contents of titanium, in order to obtain grain refinement and enhanced properties. Therefore, the effect of this combination on the microstructure and martensitic transformation temperature of each obtained alloy was analyzed. The Cu-14Al-4Ni-xTi (x= 0.5; 0.6 and 0.7 wt.%) samples were casted in an arc-melting furnace and rapidly solidified. Subsequently, all samples underwent heat treatment in a tubular furnace at a temperature of 1100ºC for 30 minutes and water quenched at 25ºC. Scanning electron microscopy (SEM) images and X-ray Diffraction (XRD) patterns showed that the presence of titanium modified the alloys microstructure, induced the formation of three titanium rich phases called "X" phase (CuNi₂Ti, Cu₃Ti and AlCu₂Ti) and reduced the presence of the brittle phase γ2 (Cu₉Al₄) for samples with 0.6 and 0.7 wt.% Ti. The titanium added to the copper based SMA also functioned as a refiner, reducing grain size up to approximately 80% with the increase of Ti content. Differential Scanning Calorimeter (DSC) results exhibited low enthalpy levels, hysteresis, as well as low start martensitic transformation temperatures. (author)